For twelve years, Chiara De Gregorio and her colleagues awoke before dawn and trudged into the rainforests of Madagascar to observe a critically endangered primate known as the indri, or singing lemur. They battled equipment-soaking downpours, bloodsucking leeches, and uncooperative lemurs, but in the end, the scientists were rewarded with a surprising discovery.
After analyzing hundreds of the primates’ songs, the scientists found that indris sing using a kind of rhythm that only people and birds have been proven to use before. It’s the first time a mammal besides humans has been found to use these rhythms, which are defined as having a set amount of time between notes.
“Indris are the only lemurs that communicate with songs,” says De Gregorio, a primatologist at the University of Turin, in Italy, and lead author of a new study on indri rhythms, published today in Current Biology.
In this case, De Gregorio refers to the animals’ tendency to harmonize in duets and even choruses of more than two animals. The animals sing to find family members that have become lost, lay claim to patches of forest, and even perform “vocal battles” with their neighbors, De Gregorio says in an email.
Of course, to the human ear, all of this sounds like a toddler with an airhorn. But within those squeaks and honks, there are patterns and note durations that overlap with qualities found in human music.
The discovery is all the more interesting considering that, though we are both primates, the last time humans and indris shared a common ancestor, there were still dinosaurs walking the earth.
Learning that people and indris share this form of rhythm “pays for all the days spent shivering under the rain, waiting for the animals to sing,” De Gregorio says.
Indris will rock you
To better understand why indri songs are special, you need to know a bit about the building blocks of music.
In 2015, a team of scientists analyzed more than 300 pieces of human-made music recorded around the globe. Across continents and cultures, certain similarities popped up again and again. The scientists found more than a dozen features common to music, including the use of particular pitches and phrase repetition.
Not surprisingly, eight of the features the scientists keyed in on had to do with rhythm. Specifically, music tends to lean on two-beat structures, also known as categorical rhythms. In these patterns, notes can be either the same length, creating a 1:1 ratio like the clicks of a metronome, or a 1:2 ratio where some notes are twice as long as the others.
To imagine a 1:2 pattern, think about Queen’s “We Will Rock You,” says Andrea Ravignani, study coauthor and a biomusicologist at the Max Planck Institute for Psycholinguistics in the Netherlands. The short-short-long pattern of stomps and claps in that song is an epic example of the 1:2 ratio.
“That is a very common rhythmic pattern in human music,” Ravignani says.
A bird called the nightingale thrush was recently shown to sing in a 1:2 ratio, but its preference was for 1:1. Now, Ravignani says the evidence is even stronger that indris use the 1:2 on the regular—much more often than the birds do.
Curiously, the lemurs would also sometimes finish their songs with a flourish known as ritardando in classical music. This is when you decrease tempo, or drag out the song’s final few notes.
“Again, this is not a very common feature,” Ravignani says. Most animals “either follow a pattern or they don't.” The fact that lemurs switch it up suggests an impressive flexibility, he says.
The discovery is all the more urgent considering the plight of indris and other lemurs. Threatened by hunting and habitat loss, experts say only between a thousand and 10,000 of the animals remain in the wild. (Listen to the music Nat Geo Explorer Ben Mirin created from the vocalizations of golden bamboo lemurs and beatbox music.)
"We are very worried about indris’ situation, which is very critical,” De Gregorio says. “They cannot survive in captivity, so once the forest is gone, they are gone too.”
Could whales be next?
While it’s surprising that indris and thrushes possess some components of music that were previously thought to be unique to humans, the discovery also begs the question of whether other singing animals, like whales, use these rhythms. (Check out this graphic that decodes whale songs.)
“To my knowledge, research like ours has never been done in cetaceans, such as whales and dolphins,” Ravignani says. “I think this kind of research, where we look for the building blocks of music in other animals, is still in its infancy.”
For all the research that’s been done on birdsong, for instance, birds’ rhythms were largely ignored until recently, says biologist Ofer Tchernichovski of Hunter College, in New York. In 2020, his team published the study on nightingale thrushes.
“We only recently discovered that in birds, and now this paper is the first one that discovered that in mammals,” says Tchernichovski, who was not involved with the new study. “My feeling is that the more people look, the more people will find it.”
For humans, a love for music is nearly universal. It can make us feel happy, sad, or romantic. It can drive us to dance, cry, or even fight. And yet, explaining what music is or why it tugs on human emotions so powerfully is incredibly difficult.
At some level, these rhythms are doing the same thing in animals, says Tchernichovski. Whether it’s about mating, competition, or kinship, the maker of each sound is trying to influence its listeners.
“Music is really some kind of a magic,” says Tchernichovski. “It has no meaning. And yet, we like it, and we engage in it, and it gives meaning to our lives.”